Beck said "indications" were that Dorner visited the homes of some of the people he was gunning for, the Los Angeles Times reported. Speaking at a news conference in Los Angeles, Beck did not go into details.
"We believe based on our investigation, Dorner did his homework," Beck said.
Dorner died from a gunshot wound last Tuesday as police laid siege to the mountain cabin he was using as a hideout in San Bernardino County. Investigators believe he took his own life after sheriff's deputies used heavy tear gas bombs to set the cabin on fire.
Dorner, a military veteran, was a police trainee in 2007 when he reported that another officer had kicked a mentally ill homeless man. He was dismissed after his report was deemed to be unfounded.
His first victims were the daughter of his lawyer and her fiance. He also killed two police officers.
Law enforcement sources told the Times Dorner knocked on the door of a police captain in January, but his wife did not let him in because she did not know him. He also allegedly visited other police officers' homes.
2014: The Year in Music [PHOTOS]